1. Field of the Invention
The present invention relates to a powder compression molding machine that compresses a powdery material to mold a medical tablet, food, an electronic component, and the like.
2. Description of the Related Art
A publicly known powder compression molding machine is provided with die bores vertically penetrating through a table, and upper and lower punches slidably held in a vertical direction above and below each of the die bores. The table and the punches are horizontally rotated together, and the upper punch and the lower punch compress a raw powdery material filled in the die bores and mold tablets when the upper punch and the lower punch pass through between upper and lower rolls.
When a product is produced using such a powder compression molding machine, there is sometimes caused binding in which a raw powdery material sticks to an inner circumferential surface of the die bore, or sticking in which the raw powdery material sticks to a tip of a punch. All of these cause flaws, roughness, chipping of the product. To prevent this kind of trouble from occurring, a powder lubricant such as metal stearate and talc is spray-coated on the inner circumferential surface of the die bore and the tip of the punch in advance, and thereafter the raw powdery material is filled in the die bore to be tabletted (see, for example, Japanese Patent Application Laid-Open as No. 2010-017765).
A spray device for spraying the lubricant is divided into a downward spray portion for spraying the lubricant toward the inner circumferential surface of the die bore and the tip of the lower punch, and an upward spray portion for spraying the lubricant toward the tip of the upper punch. Both of the spray portions are provided with a DC high-voltage electrode in the vicinity of a spray port. The lubricant sprayed from the spray port is electrostatically charged and sticks fast to the inner circumferential surface of the die bore, a tip face of the tip of the lower punch, and a tip face of the tip of the upper punch. The lubricant thus stuck does not fall off by a cause such as vibrations generated by vertical movements of the punches or a wind pressure generated by a high-speed rotation of the table, but is transferred and sticks to molded articles from the die bore and the tips when the raw powdery material is compressed and molded by the punches.
A sum of areas of the inner circumferential surface of the die bore and the tip face of the tip of the lower punch is larger than an area of the tip face of the tip of the upper punch. Accordingly, an amount of the lubricant applied to the inner circumferential surface of the die bore and the tip face of the tip of the lower punch is larger as compared with an amount of the lubricant applied to the tip face of the tip of the upper punch. In view of this, conventional control is performed to absorb predetermined amounts of the lubricant individually onto the inner circumferential surface of the die bore and the tip face of the tip of the lower punch, and onto the tip face of the tip of the upper punch by differentiating an intensity of charging in the downward spray portion and an intensity of charging in the upward spray portion from each other, while equal amounts of the powder lubricant are individually supplied to the downward spray portion and the upward spray portion from an external lubricant supply device.
However, according to such control, two electrostatic charging devices with application voltages different from each other should be installed, which increases a cost. In addition, the fact that an excess amount of the lubricant that may not be adhered onto the tip of the upper punch is sprayed from the upward spray portion is undeniable.